A field effect transistor (FET) is a device in which regions called a source and a drain are provided in a semiconductor, each of the regions is provided with an electrode, potentials are supplied to the electrodes, and an electric field is applied to the semiconductor with the use of an electrode called a gate through an insulating film or a Schottky barrier so that the state of the semiconductor is controlled, whereby current flowing between the source and the drain is controlled. As the semiconductor, Group IV elements (also referred to as Group 14 elements) such as silicon and germanium, Group III-V compounds such as gallium arsenide, indium phosphide, and gallium nitride, Group II-VI compounds such as zinc sulfide and cadmium telluride, and the like can be given.
In recent years, FETs in which oxides such as indium oxide (Patent Document 1), zinc oxide (Patent Documents 2 and 4), and an indium gallium zinc oxide-based compound (Patent Document 3) are used as semiconductors have been reported. In a FET including such an oxide semiconductor, relatively high mobility can be obtained, and such a material has a wide bandgap of greater than or equal to 3 eV; therefore, application of the FET including an oxide semiconductor to displays, power devices, and the like is examined.
To be specific, it is reported that the field effect mobility of a FET including zinc oxide or an indium-gallium-zinc-oxide-based compound is 20 cm2/Vs at most, while the field effect mobility of a FET including indium oxide as a main component is 50 cm2/Vs or higher. It is empirically clear that a higher field effect mobility can be obtained with a higher ratio of indium in an oxide.
In general, an oxide semiconductor including zinc or indium as a main component (here, “the main component” refers to an element accounting for 50 at. % or more of all elements having an atomic number of 11 or more in the oxide semiconductor) and showing a p-type conductivity has not been reported so far. Accordingly, a FET using a PN junction like a FET including silicon has not been reported. As disclosed in Patent Documents 1 to 4, a metal-semiconductor junction in which a conductive electrode is in contact with an n-type or i-type (in this specification, “an i-type semiconductor” refers to a semiconductor having a carrier concentration of lower than or equal to 1×1014 /cm3) oxide semiconductor has been used for forming a source and a drain.
FIG. 7A illustrates an example of a conventional FET including an oxide semiconductor. Here, a gate insulating film 14 is provided in contact with one surface of a semiconductor layer 11 including an oxide semiconductor, and a gate 15 is provided over the gate insulating film 14. A source electrode 13a and a drain electrode 13b are provided on the other surface of the semiconductor layer 11.
The thickness of the semiconductor layer 11 has not been particularly considered in many cases. In addition, as a material of the gate insulating film 14, silicon oxide, silicon nitride, or the like has been used, and the thickness of the gate insulating film 14 has not been considered particularly as well. A material of the source electrode 13a and the drain electrode 13b has not also been considered particularly, and titanium, molybdenum, and the like have been reported.
In practice, a protective insulating film 16 is provided in contact with the semiconductor layer 11 as illustrated in FIG. 7B. As a material of the protective insulating film 16, a material that can be used as a material of the gate insulating film 14 is used.
In a FET, it is generally preferable that an ohmic contact is formed in a contact portion between a source electrode and a semiconductor layer or a contact portion between a drain electrode and a semiconductor layer. For this purpose, the material of the source electrode 13a and the drain electrode 13b is preferably a material having a work function lower than the electron affinity of an oxide semiconductor that is used for the semiconductor layer 11. For example, work functions of titanium and molybdenum are lower than the electron affinity of indium oxide (approximately 4.8 eV) and thus preferable in terms of forming an ohmic contact.
Further, in portions where the metal is in contact with the semiconductor layer 11, electrons are injected from the metal to the semiconductor layer 11, so that the concentration of electrons in the semiconductor layer 11 is increased, which couples regions having high electron concentration together particularly in the case of a short-channel FET having a channel length (the distance between the source electrode 13a and the drain electrode 13b) of 0.3 μm or less, and is a factor of a reduction of FET characteristics (e.g., a negative shift of the threshold voltage, an increase in S value, and a phenomenon in which current flows between a source and a drain in an off state (off-state current)).
In a FET in which a source and a drain are formed using a metal-semiconductor junction, a higher carrier concentration of a semiconductor causes a larger off-state current. In other words, even when the source-gate voltage (hereinafter referred to as a gate voltage) is 0 V, a substantial amount of current (hereinafter referred to as a drain current) flows between the source and the drain (this FET characteristic is called “normally on”). For this reason, it is expected that the off-state current is reduced by reducing the concentration of carriers in the semiconductor so that the semiconductor is formed to be an i-type semiconductor and that the drain current at a gate voltage of 0 V is 1×10−9 A or lower, preferably 1×10−12 A or lower, and further preferably 1×10−15 A or lower.
However, oxygen deficiency is likely to be caused in indium oxide or an oxide semiconductor including indium as a main component, and it has been difficult to set the carrier concentration to 1×1018/cm3 or lower. Accordingly, a FET including an oxide semiconductor including indium as a main component has high mobility but is normally on, and this tendency becomes more significant as the concentration of indium becomes higher. For example, in the case of using indium oxide, the drain current cannot be 1×10−9 A or lower unless the gate voltage is set to be −10 V or lower.